Setting Aux Heat by time of day?

[Tom Campbell]

Here in the PPL service area of Pennsylvania, the electric company has just instituted a new Time of Use (TOU) pricing policy - 6.108 cents off peak, 7.5 on peak (and that is supposed to include transmission charges too!) That makes the off peak price, even with resistance coils, about the same as oil and even lower than propane (there's no NG in our area). So it got me to thinking:

Since I've been b*tching so much here about defrost noises, why not just set the thermostat to Aux Heat overnight since rates are so low and keep the heat pump from coming on at all? Sure, it's a little more expensive than running the compressor, but at 6.1 cents / Kwh, it's not really more than what my oil-heat neighbors are paying. And aux heat is warmer and more comfortable to the skin in the bathroom when you get in the shower in the morning too.

I was really hoping to use the programmable feature of my Honeywell VisionPRO IAQ to accomplish this automatically, but alas, setting AUX HEAT is not one of the things you can control in the scheduling. I assume it's the same for all the other Heat Pump compatible thermostats out there (Although I'm all ears if anyone has any models that can).
- BTW - The house is pretty big and has 2 zones, each designed identically with a VisionPRO IAQ, Goodman MBVC1600 air handler and GSZC18 compressor. The house is quite well insulated - even at 9 degrees outside the HP alone has been able to keep the house at 70 degrees.

Are there any simple ways to force the system to use Aux Heat only from 10PM to 7AM?

The only thing I've been able to think of, and it seems overly complex, is:

Since I have a thermostat that can use an external sensor for set-point control, would it be possible to use the C7089 Outdoor Sensor (that can be added to the VisionPRO) to simulate the time-base aux heat use?

The approach would be use a small timer with an open/close circuit to in line with the wiring of the C7089 to the VisionPRO controller (I have to extend the connection anyway since the 5 foot lead on the sensor is not long enough). From 10PM to 7:30 AM the circuit would go open, creating infinite resistance (indicating colder than -40F). The rest of the time it would go closed, allowing the sensor to read normally. And of course, it would require entering a set point in the thermostat settings so the outdoor sensor would be incorporated into the calculations.

Alternately, since making the circuit go open might make the thermostat think the sensor was no longer connected, I may need to have it go through a resistor equal to the -30 or -40 temp would create. That's going to be much more difficult.

I really think using the outdoor sensor in this way, while imaginative, is probably not the best way to accomplish this.

I welcome suggestions for better ways to force Aux Heat only from 10PM to 7AM - whether they be for a specific alternate thermostat, a third-party timer my installer can add to the system, or well, whatever.

[kb3ca]

Why don't you just switch your thermostat to"emg heat" at 10pm and back to "heat" at 7am?

[Tom Campbell]

Why don't you just switch your thermostat to"emg heat" at 10pm and back to "heat" at 7am?

For the same reason people buy setback thermostats. Because remembering to change the setting every morning and night is a pain and very likely to be forgotten.

[Tom Campbell]

After a bit more reading, perhaps the easier (somewhat) approach is to look at wiring it like an "Old School" outside balance point thermostat would be set up - except trigger based on a clock instead of an outside thermometer switch?

[garya505]

I like the resistor idea on the outdoor sensor. Just use your timer to short out the resistor in the day time. I believe the resistance tables are available. Looks like about a 50k resistor would do it.

[Tom Campbell]

I like the resistor idea on the outdoor sensor. Just use your timer to short out the resistor in the day time. I believe the resistance tables are available. Looks like about a 50k resistor would do it.

According to the specs -20F is produces 107K, -10F is 80K, so a 100K should do the trick. I'd imagine a 1/2 watt resister would be OK. I guess I could cycle it with a normally open relay running off a 12 volt transformer, and then just put the transformer on a timer. (normally open relay in case the setup fails, it would default to normal operation)

[Southern Mech]

I Am trying to figure out, just cause the power is cheap, why would you want to burn the aux heat, and use 3 times as many kw vs the cheap heatpump?

[jeepgrady]

I Am trying to figure out, just cause the power is cheap, why would you want to burn the aux heat, and use 3 times as many kw vs the cheap heatpump?

I was thinking the same. Running the HP is still much cheaper than the strips.

[garya505]

According to the specs -20F is produces 107K, -10F is 80K, so a 100K should do the trick. I'd imagine a 1/2 watt resister would be OK. I guess I could cycle it with a normally open relay running off a 12 volt transformer, and then just put the transformer on a timer. (normally open relay in case the setup fails, it would default to normal operation)

Ya, 100k would do it. 1/2W is probably gross overkill but would work.

If my Prestige IAQ would work with a wired sensor I might even try it myself.

[SJProwler]

I'm doing the same thing with my Heat Pump on my new system. Off peak is .034 per kwh and on peak is .148 per kwh delivered price. Since I have an iComfort thermostat with my new system I'm using my old programmable as a timer to lock out the HP during on peak. I have to switch the outdoor temp sensor out and resistor in with a relay to fool the system it's below the balance point. Just removing the sensor probably won't work since then the system will just think there isn't one.

[Tom Campbell]

I Am trying to figure out, just cause the power is cheap, why would you want to burn the aux heat, and use 3 times as many kw vs the cheap heatpump?

I was thinking the same. Running the HP is still much cheaper than the strips.

I know. It sounds really stupid doesn't it - like throwing money away.

But my logic 2-fold:

1: Noise: I was stupid and didn't realize how loud the defrost cycle could be - it wasn't even something I considered when buying a heat pump to replace the old one (we bought this house over the summer). But what is done is done. I can still move it further from the bedroom window, but I think even on the far side of the house I will hear it at night. It only bothers me at night - I don't mind it during the day.

2: Comfort: As the outside temp drops below 20, the COP on the heat pump is still great, but the duct air is a bit cool to the skin. My wife doesn't like having to cover up at night - and she doesn't like a cool draft in the bathroom at night or in the morning. And when the compressor does go in to defrost, the duct air is a bit colder than room temp - I hear about it every night! It was important enough that I looked hard at investing in a new oil or propane system like my neighbors' whose homes feel "toasty". Yet when I did the math on the cost to run the resistive coils at the new PPL rate, I found it about the same operating cost as the oil and much lower than the propane. And since it's only overnight, my overall cost to heat is still much lower than my neighbors'.

So yes, I am kind of throwing away money. But if I can make it work I think it will be worth it.

[garya505]

Too bad even the high-end stats don't have a feature like this. Seems like it wouldn't be to difficult to do in software in a stat.

[Tom Campbell]

Ya, 100k would do it. 1/2W is probably gross overkill but would work.

If my Prestige IAQ would work with a wired sensor I might even try it myself.

I'm doing the same thing with my Heat Pump on my new system. Off peak is .034 per kwh and on peak is .148 per kwh delivered price. Since I have an iComfort thermostat with my new system I'm using my old programmable as a timer to lock out the HP during on peak. I have to switch the outdoor temp sensor out and resistor in with a relay to fool the system it's below the balance point. Just removing the sensor probably won't work since then the system will just think there isn't one.

I did a quick test to see what would happen if I tried setting the HW IAQ to think it had an outdoor sensor even though none was attached (infinite resistance) - in hopes that it would just ignore it (setting 342). That would have allowed me to just hook up the resistor on a timer and leave it open other times. No luck. It throws an error 53 and flashes on and off obnoxiously, so I'll have to either use a relay to toggle between 2 different resistors (high temp and low temp) or install an actual HW outdoor sensor and toggle between it and the 100K resistor. It may be another week before I get the time to pick up parts and give it a try.